The Hammer-Heads are mentioned in a few subsequent books. In Emerald City, they're listed among the dangers of Oz, and also called the "Wild People." At the beginning of Patchwork Girl, Ojo tells Unc Nunkie, "All I've ever seen of the great Land of Oz, Unc dear, is the view of that mountain over at the south, where they say the Hammerheads live--who won't let anybody go by them--and that mountain at the north, where they say nobody lives." This actually seems rather unlikely, however. While the map of Oz on the Tik-Tok endpapers shows Ojo's forest home as being in the southern Munchkin Country, a few references suggest that it's actually in the northern part. Not only is Dr. Pipt's mountain said to border on the Gillikin Country, but Ojo and his companions first meet the Scarecrow when he's on his way to visit Jinjur, who is shown by both the map and the events of Tin Woodman to live northeast of the Emerald City. But even we accept the map's location, it doesn't look anywhere near close enough to the Hammer-Heads' mountain for Ojo to see it from his forest dwelling. Then again, maybe the Hammer-Heads colonized another mountain, and that's what Ojo is referring to. And Jack Pumpkinhead, in his own book, tells Peter Brown that Kuma Party had recently assisted Ozma in subduing the Hammer-Heads, with no additional details given.
One popular myth involving the Hammer-Heads is that they're so territorial because they're protecting something valuable. I'm not sure how this idea started, but I've seen it show up quite a few times. I don't see anything in the Wizard text to suggest that they have any secrets, rather than just an ornery attitude toward strangers. Still, the idea of the secret of the Hammer-Heads is rather interestingly used in Onyx Madden's Mysterious Chronicles. In this book, they were charged by Queen Lurline with guarding the access to the OPALOZ, which provides the life-blood of Oz. Only a person bearing a primrose is allowed to pass.
Considering what we learn of the Hammer-Heads in Wizard, it's actually kind of odd that they don't show up more often than they do. Not only is it necessary for Dorothy to summon the Winged Monkeys to cross the hill and reach Glinda's palace, but Glinda herself tells Dorothy's companions that she'll need to employ the Monkeys herself in order to get them past the Hammer-Heads. In Land, however, Glinda's army marches from her palace to the Emerald City with no interference from Hammer-Heads mentioned, and later books follow suit in having characters journeying to or from Glinda's without having to contend with the armless people. It seems to me that the most likely explanation is that Glinda found an alternate path to the north, whether through magic or simply knowledge of the nearby terrain. In one of my own unfinished manuscripts, there's a tunnel through the Hammer-Heads' mountain, with a schism resulting between the more conservative inhabitants who still want to keep everyone away, and the liberal ones who don't mind strangers passing UNDER their territory. I'm also kicking around an idea for a tale about the long-lost King of the Hammer-Heads, which is actually based on something my brother and I made up years ago.